1. Field of the Invention
This invention relates to an image processing apparatus and image processing method which form synthesized image information by embedding and synthesizing additional sub-information (such as security information) set in an invisible state into main image information (such as a face image of a human being) set in a visible state.
2. Description of the Related Art
In recent years, greater importance is attached to the technique for making an electronic watermark or electronic signature in order to prevent forgery or falsification of an image as information is more electronized and the Internet is more widely used. The electronic watermark technique is a technique for embedding additional sub-information (sub-image information) in an invisible state into main image information. For example, the electronic watermark, technique is utilized for a personal authentication medium or work on which personal information such as an IC card is recorded. By use of the electronic watermark technique, it is possible to take a countermeasure against illicit copying, forgery or falsification of a personal authentication medium or work and protect personal information of the personal authentication medium and copyright of the work.
For example, as the electronic watermark technique, a technique for embedding sub-image information in main image information by use of the characteristic of a color-difference component or high spatial frequency component which is difficult to be sensed by a human being.
Further, as an apparatus which records synthesized image information on a recording medium, for example, a recording apparatus of sublimation type thermal transfer recording system or melting type thermal transfer recording system is used.
Generally, in the sublimation type thermal transfer recording system, a material which can be dyed with a sublimation material is limited. Therefore, in the sublimation type thermal transfer recording system, recording media which can be applied are limited and the degree of-freedom of selection of recording media on which an image is recorded is low. Therefore, in the sublimation type thermal transfer recording system, recording media which can be used are limited and the degree of security thereof is lowered in many cases. Further, a sublimation dye is generally low in image durability such as solvent resistance and light resistance.
On the other hand, in the melting type thermal transfer recording system, a coloring material which is generally said to have high light resistance can be selected. Further, in the melting type thermal transfer recording system, the degree of freedom of selection of a recording medium is high. Further, in the melting type thermal transfer recording system, since a recording medium having a highly special property can be used, the degree of security can be enhanced. However, in the melting type thermal transfer recording system, a dot area gradation method which records gradation by changing the sizes of transferred dots is used. Therefore, the melting type thermal transfer recording system has a problem that it is difficult to attain the same gradation performance as that of the sublimation type thermal transfer recording system.
As a countermeasure against the above problem, a method (which is hereinafter referred to as an alternate driving/recording method) for arranging and recording transfer dots in a zigzag form is disclosed.
Further, it is assumed that the electronic watermark technique basically deals with digital data. Therefore, in an image recording apparatus such as a color printer which prints synthesized image information on a recording medium, sub-information embedded in the synthesized image information can be prevented from being destroyed or altered.
For example, in Jpn. Pat. Appln. KOKAI Publication No. H6-59739, a recording method of melting type thermal transfer recording system of an image recording apparatus which enhances the gradation recording performance is disclosed. However, in the recording method disclosed in Jpn. Pat. Appln. KOKAI Publication No. H6-59739, data of zigzag form is thinned out from recording image data and information of a corresponding portion is lost. Therefore, if synthesized image information having sub-information embedded in main image information is recorded on a recording medium by using the technique disclosed in Jpn. Pat. Appln. KOKAI Publication No. H6-59739, there occurs a problem that the sub-information embedded in the synthesized image information will be destroyed.
Further, in Jpn. Pat. Appln. KOKAI Publication No. H9-248935, a technique for embedding sub-information in image data by use of the characteristic of a color-difference component or high spatial frequency component which is difficult to be sensed by a human being is disclosed. However, in the method disclosed in Jpn. Pat. Appln. KOKAI Publication No. H9-248935, only contour information of the sub-information is held in the main image information. Therefore, in the method disclosed in Jpn. Pat. Appln. KOKAI Publication No. H9-248935, only contour information can be held and concentration information cannot be held. As a result, the method disclosed in Jpn. Pat. Appln. KOKAI Publication No. H9-248935 is not suitable for a restoration process or a process for embedding two-dimensional plane information such as a two-dimensional bar code.